1. Field of the Invention
This invention relates to the protection of underground or immersed structures exposed to wet corrosion.
In industrial practice many metal structures suffer from this technical problem, for example underground fuel storage tanks, off-shore structures, fluid transporting pipelines such as water, gas and oil pipelines, and other infrastructures for industrial, civil or military use.
The present invention is described with reference to underground pipelines, this constituting one of its most difficult and interesting applications, but without in any way limiting it to this specific application.
2. Description of the Prior Art
Liquid or gas transporting pipelines, such as gas, water or oil pipelines, are formed by welding together lengths of metal pipe, generally of steel, the continuous pipeline assembled in this manner being laid in its final seat, generally consisting of a sufficiently deep trench, and then covered in order to recreate the pre-existing environment and hence not hinder its further use. The assembled continuous pipeline is provided with protection against wet corrosion, as the environment in which the underground or immersed pipeline lies is very aggressive towards ferrous materials.
The integrity and preservation of the pipeline during its entire operating life are very important not only because of its high construction cost but also, and in particular, because fluid leakages must be prevented both because of their economical aspect and because they must not be allowed to cause danger, pollution or serious disturbance. The protection generally used consists of two different types of protection in mutual cooperation, namely "passive" protection consisting of a covering which isolates the pipeline from the surrounding environment and "cathodic" protection which applies an electric potential for inhibiting possible electrochemical reactions which could attack the pipeline metal to the extent of forming a through hole.
The "passive" protection is generally formed by applying a continuous covering to the properly cleaned pipeline. This covering can be of two main types, the first type being coverings of hydrocarbon materials such as asphalts, bitumens and fats which are generally applied hot in a number of layers together with fibrous material reinforcements, and the second type being synthetic polymer coverings such as polyethylene, polyvinylchloride and epoxy polymers, these being applied in the form of strips wound spirally about the pipeline and superimposing the side edges, or by die-casting. Web, felt or card made of glass wool, asbestos or other non-rotting fibrous material are used as protection and reinforcement.
This protection is not on its own sufficient to protect an underground or water-immersed pipeline for the required time. In this respect, the following should be noted:
no material is free of a certain porosity and permeability, even if perfectly applied, and hence a certain passage of the chemical species responsible for corrosive attack takes place in any event through the protective layer, even if very slight;
the sequence of operations involved in the preparation, covering, lifting, laying and burying of the pipeline can result in immediate slight damage or imperfections to the applied covering, these defects then triggering corrosion phenomena;
the hydrocarbon or polymer materials and their reinforcements have a chemical stability which is very high but is not absolute, particularly in relation to temperature or humidity changes;
natural phenomena, such as earthquakes, landslips or floods, and accidental events can damage the pipeline passive protection.
The "cathodic" protection protects the pipeline at those points in which porosity, damage or imperfect application of the covering have left the metal surface exposed to corrosive attack.
The variation in the state of the passive protection covering on the pipeline can be monitored by the method described in European patent application Publ. No. 0,411,689 and U.S. patent application Ser. No. 07/557,968 now abandoned in the name of the present applicant.
According to this method, the cathodic protection current applied to the pipeline is modulated by square waves. The application of square waves signals enables the resistive components and the capacitive components constituting the overall impedance of the pipeline-earth system to be separated and determined.
The variation in the ohmic resistance of an underground pipeline, deriving from progressive ageing of the covering, from distributed microcorrosion and from variations in earth conductivity, shows a continuous pattern of fall-off to lower values. The effects produced by landslip follow the same pattern.
This method is aimed at monitoring the state of the covering but 7ithout involving repair, the most that can be done being to increase the cathodic protection using greater intensity currents to compensate the progressive reduction in impedance.
Instead, the present invention monitors accidental events which can damage the pipeline or rather its protective covering, for example in the case of underground pipelines any accidental impact by excavation equipment, which would cause a sudden deterioration concentrated within a section of the pipeline.
In the case of pipelines immersed in water, such accidents can be caused by anchors or anchoring chains, fishing equipment which trawls the water bed, watercraft which run aground in shallow water, dragging equipment etc.
In contrast to normal covering decay, this sudden concentrated damage requires urgent local repair, otherwise it could cause rapid decay of the entire pipeline.
From the variation in the ohmic loss RI of the pipeline shown in FIG. 1 it can be seen that if the cathodic current applied to the pipeline is permanently modulated (in accordance with diagram A) symmetrically about the desired current by means of square waves, the wave form of the voltage induced in the pipeline-earth system comprises rounded crests and cavities for the capacitive components and a vertical level difference RI for the ohmic components, reflecting the state of the pipeline covering. According to the cited European Patent Appln. 0,411,689 the state of the covering is monitored by using cathodic protection currents at low modulation level and in any event below 10%, the preferred modulation range being between 0.5% and 5%. The modulation frequency is less than 100 Hz, the preferred range being between 0.05 and 1.0 Hz, and the ratio of the two half-periods is between 0.001 and 1000, the preferred range being between 0.1 and 10.
The insulation resistance (R) offered by the covering is determined according to the cited patent application by measuring with a recording voltmeter the variation in the potential due to the ohmic loss (.DELTA.V*), in accordance with diagram B, corresponding to the applied modulating current (I*), where I* is the difference between the currents I.sub.max and I.sub.min. Then: EQU R=.DELTA.V*/I*
This enables the insulation resistance R of the pipeline covering to be determined, this being a measurement of its integrity. Knowing the value of R, the effective protection potential (V.sub.p) can be calculated once its apparent overall potential (V.sub.a) and the total cathodic protection current (I) are known, using the formula: EQU V.sub.p =V.sub.a -RI
In the diagram of FIG. 1, the variation in the phenomenon shown on the left side of the diagram corresponds, as described up to this point, to the "natural" decay of the covering of the pipeline monitored by the monitoring system of European Patent Appln. 0,411,689.
In the absence of accidental events, this natural decay results in a slow continuous reduction in the value of RI.
The central part of the diagram of FIG. 1 shows the variation arising on accidental contact between a metal body and the metal part of the pipeline. It can be seen that the value of RI suddenly falls, whereas the wave crests and depressions, which correspond to the capacitive components, show substantially no modification. This pattern reflects the fact that the metal body which makes contact with the metal part of the pipeline acts as a conductor to earth.
The right side of the diagram of FIG. 1 shows the variation in the phenomenon when contact between the metal body and the pipeline ceases. It can be seen that the value of RI suddenly rises, but to a value of less than that prior to contact. This variation reflects the fact that the metal body no longer acts as a conductor, but that it has damaged the covering to a certain extent.
This accidental event therefore produces a phenomenon which is not typical of natural covering decay, namely a sudden fall in the ohmic resistance followed by a rise to a value dependent on the area of the damage.